The bovine placenta produces considerable amounts of pregnancy-associated estrogens, predominantly estrone sulfate, which does not interact with classical nuclear estrogen receptors. Thus, bovine placental estrogens may rather act as local regulatory factors than as hormones in a classical sense. To obtain information on the local availability of free estrogens in bovine placentomes, the expression pattern of the estrogen specific sulfotransferase SULT1E1 was characterized using antisera against the bovine and human enzyme, respectively. In western blot both antisera detected a band of the expected molecular weight (approx. 33 kDa) in placentomes and fetal liver. In immunohistochemistry the two antisera yielded virtually identical results. In placentomes distinct staining was restricted to the cytosol of uninucleated trophoblast cells (UTC). The staining pattern in UTC, immature and mature trophoblast giant cells (TGC) is consistent with a down-regulation of SULT1E1 during TGC differentiation. During early and midgestation staining intensity in UTC was higher in the trophoblast covering the chorionic plate and basal parts of stem villi compared to other regions of the villous tree, whereas during late gestation and at parturition an almost ubiquitous distinct staining of UTC was found. Correspondingly, relative SULT1E1-mRNA levels as measured by quantitative real-time RT-PCR increased significantly during late gestation (p = 0.0043). Comparative measurements showed that SULT1E1-mRNA levels in adult bovine organs were considerably lower compared to placentomes and fetal liver. The results suggest that the activities of free estrogens produced in bovine TGC are curtailed by SULT1E1 expressed in UTC and in fetal liver. Bovine pregnancy-associated estrogens produced in trophoblast giant cells are predominantly sulfonated in uninucleated trophoblast cells.